This invention relates to apparatus for treating a biological target by delivering focused ultrasound at high intensity to a focal point. It further relates to a method for adjusting the frequency of apparatus for treating a biological target by delivering high intensity focused ultrasound to a focal point.
The invention falls in the field of tissue treatment using focused ultrasound, and more particularly the field of tissue destruction inside an organism by creation of high temperatures using focused ultrasound.
In the general field of focused ultrasound, as the person skilled in the art knows, various types of treatment can be distinguished: the treatment that has been around longest is treatment by lithotripsy, which applies to destroying hard bodies; this type of treatment uses shock waves, i.e. short high power pulses. Later, it was proposed to treat soft issue by hyperthermia, by heating tissue to slightly elevated temperatures, i.e. less than 45xc2x0 C. Hyperthermia involves emitting ultrasound in the form of long and lower power pulses to the tissue to be treated. Finally, currently, treatment of soft tissue using high intensity focused ultrasound, generally called HIFU (high intensity focused ultrasound) is being proposed. HIFU treatment involves heating tissue to elevated temperatures, typically greater than 45xc2x0 C.
These various types of treatment involve very different technical problems, both as regards sending and focusing of the ultrasound as well as its propagation.
HIFU treatment raises different problems. Generally speaking, the aim is to improve effectiveness of treatment, i.e. destruction of selected tissue. For this, the first problem resides in a suitable choice of the ultrasound transmission parameters; the latter, and in particular ultrasound frequency, need to be chosen very accurately. They generally depend on numerous factors such as: target depth, nature of tissue, type of necrosis desired.
A second problem is that of gaining access to the targets or tissue to be treated. Due to patient anatomy, targets are sometimes difficult to get at for ultrasound beams. Moving the transducer has been proposed; moving the transducer may however also be limited by patient morphology. In prostate treatment by endorectal probe, various solutions to this problem have been proposed, see for example French Patent applications serial numbers 9102620, 9309158, 9608096, 9401304, 9406539. These various solutions could still be improved, for ensuring better treatment, in precise areas, by hyperthermia or HIFU.
A third problem resides in the fact that the beam emitted by an ultrasound focused transducer is generally effective within a fixed region, called the focal zone. Now, this focal zone most frequently is smaller than the size of the target tissue. Treatment of extensive targets is consequently a problem. One proposition was to successively employ, by endorectal route, transducers of various focal lengths, for example, in the case of the prostate, a first one, of short focal length, suitable for treating the posterior region and another one, of longer focal length, for the anterior region. This method involves changing the probe during the session, which is not desirable.
One proposed solution to this third problem consisted in employing variable focal length transducers. These can be constructed from an array of individual transducers. Do-Huu was the first to employ annular arrays for hyperthermia ((JP Do-Huu, P Hartmann, Annular array transducer for deep acoustic hyperthermia, IEEE Ultrasonics Symp, Vol 81CH1689-9, pp. 705-710, 1981, or U.S. Pat. No. 4,586,512 of May, 1986). Still for hyperthermia, we can cite the work of Cain (C A Cain, S A Umemura, Concentric-ring and sector vortex phased array applicators for ultrasound hyperthermia therapy, IEEE Trans Microwave Theory Tech, vol MTT-34, pp 542-551, 1986) and the work of Ebbini (E S Ebbini, C A Cain, A spherical-section ultrasound phased array applicator for deep localized hyperthermia, IEEE Trans Biomed Eng, vol 38, pp 634-643, 1991).
J Y Chapelon et al, The feasibility of tissue ablation using high intensity electronically focused ultrasound, IEEE Ultrasonics Symp, Vol 93CH3301-9, pp 1211-1214, 1993 proposed using annular phased arrays for HIFU.
The work of Hynynen and corresponding publications, for example K Hynynen et al, Feasibility of using ultrasound phased rays for MRI monitored noninvasive surgery, IEEE Trans UFFC, Vol 43, No. 6, 1996 proposes HIFU treatments.
A variable focal length transducer can also be constructed using a fixed focus transducer and an acoustic lens, as disclosed in French patent 2,715,822 in the name of Dory.
In every case, it is essential to adapt treatment parameters to target depth for obtaining satisfactory therapeutic effect. In particular, the operating frequency of the transducer must be determined. This is calculated from the equation giving absorbed acoustic power per unit of volume (W/cm3) at the focus of a focused transducer:
Q=2xcex1FI0Gexe2x88x922xcex1Fdxe2x80x83xe2x80x83(1)
where:
Q is the acoustic power absorbed per unit of volume
xcex1 is the acoustic attenuation factor (Neper/cm/MHz)
I0 is the acoustic intensity at the transducer emission surface (W/cm2)
G is antenna gain
F is frequency (MHz)
d is the thickness of the absorbing medium (cm) as explained in Hill C. R. Optimum acoustic frequency for focused ultrasound surgery in Ultrasound in Med and Biol; 20; 271-277; 1994 and Lesion development in focused ultrasound surgery: a general model in Ultrasound in Med and Biol; 20; 259-269; 1994.
This formal approach is known and employed by designers of apparatus for tissue treatment by focused ultrasound, for determining optimum operating frequency of a therapy transducer depending on depth or acoustic attenuation of the intended target. This choice is defined a priori and remains fixed for a given transducer.
Seppi, in U.S. Pat. No. 4,875,487 discloses, for hyperthermia, use of wideband transducers and choice of working frequency range depending on target depth. That Patent additionally proposes employing a wideband signal so as to create incoherent beams which are, consequently, unfocused.
European patent application 0,351,610 discloses wideband transducers focused electronically, focusing being controlled as a function of cavitation.
This invention proposes an elegant and simple solution to the problem of distributing acoustic power in ultrasound treatment; it ensures better control of overall power, and good definition of the region treated.
More precisely, the invention provides apparatus for treating a biological target by emitting high power focused ultrasound towards a focal point, comprising wideband transducer means for emitting ultrasound, means for controlling said transducer means for emitting focused ultrasound over a narrow frequency range, and means for adjusting the frequency range of said controlling means as a function of measurement results.
According to one embodiment of the apparatus, the means for emitting ultrasound have a variable focal length.
According to a further embodiment, the apparatus additionally comprises coupling means of variable thickness adjacent to said ultrasound emitting means.
The ultrasound emitting means preferably have a fixed focal length and the apparatus can additionally comprise variable thickness coupling means adjacent to the emitting means.
According to one embodiment, the apparatus additionally comprises means for measuring acoustic attenuation in the region of a focal point, the means for adjusting frequency range performing adjustment of the focused ultrasound frequency range as a function of results supplied by acoustic attenuation measurement means.
The apparatus preferably comprises means for measuring mean acoustic attenuation variation close to a focal point, and the means for adjusting frequency range performing adjustment of a focused ultrasound frequency range as a function of results supplied by the means for measuring mean acoustic attenuation variation.
According to one embodiment, the apparatus additionally comprises means for calculating or measuring temperature in the region of a focal point, and the means for adjusting frequency range perform adjustment of the focused ultrasound frequency range as a function of results supplied by said means for calculating or measuring temperature.
According to a further embodiment, the apparatus comprises means for determining the thickness of. tissue through which ultrasound has passed, and the means for adjusting frequency range perform adjustment of the focused ultrasound frequency range as a function of results supplied by said thickness-determining means.
The means for determining a thickness of tissue through which ultrasound has passed preferably comprises means for measuring thickness of variable-thickness coupling means.
According to one embodiment, the apparatus comprises means for calculating a displacement of a lesion as a function of time of shooting, and for calculating thickness through which ultrasound has passed, the adjustment means performing focused ultrasound frequency adjustment as a function of of displacement and thickness.
According to a further embodiment, the apparatus comprises means for calculating lesion depth as a function of shooting time, the adjustment means performing adjustment of focused ultrasound frequency as a function of depth.
In one embodiment, the adjustment means perform adjustment of frequency range before a shot.
In a further embodiment, the adjustment means perform frequency range adjustment during a shot.
A method for adjusting the frequency of apparatus for treating a biological target by emitting high intensity focused ultrasound towards a focal point is provided, the method comprising the steps of:
measuring attenuation or variation in attenuation of a biological target; and
adjusting focused ultrasound frequency as a function of measured attenuation.
A method for adjusting the frequency of apparatus for treating a biological target by emitting high intensity focused ultrasound towards a focal point is also provided comprising the steps of:
measuring a thickness of tissue through which ultrasound has passed; and
adjusting focused ultrasound frequency as a function of this thickness.
Measurement of tissue thickness through which ultrasound has passed can comprise the steps of:
calculating a focal length between an ultrasound sender and a focal point;
measuring the distance between the sender and a first interface with a body containing the target; and
subtracting a distance between the sender and the first interface from focal length in order to obtain a thickness of tissue through which ultrasound has passed.
Ultrasound frequency adjustment is preferably performed so as to apply a given power Q to a target.
Ultrasound frequency adjustment is advantageously performed by application of the following formula:
Q=2xcex1FI0Gexe2x88x922xcex1Fdxe2x80x83xe2x80x83(1)
where:
Q is the acoustic power absorbed per unit of volume
xcex1 is the acoustic attenuation factor (Neper/cm/MHz)
I0 is the acoustic intensity at the transducer emission surface (W/cm2)
G is antenna gain
F is frequency (MHz)
d is the thickness of the absorbing medium (cm).
In one embodiment of the method, the step of frequency adjustment is performed before a shot.
In another embodiment, the step of frequency adjustment is performed during a shot.
Further characteristics and advantages of the invention will become clear from the description which follows of some embodiments, provided by way of example and with reference to the attached drawings.